The present invention generally relates to intravascular stents and more particularly pertains to covers that permanently envelop the stent""s supporting structure.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain patency. These devices are typically intraluminally implanted by use of a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the body lumen. Once in position, the stent is deployed which, depending on its construction, is achieved either automatically by for example the removal of a restraint or actively by for example the inflation of a balloon about which the stent is carried on the catheter.
Various stent configurations are well known and typically comprise a generally tubular arrangement of struts, spines, wires, etc. Such elements are advantageously positioned, oriented and interlinked to enable the stent to be expanded and to then provide the required radial support to lumen walls in which they are deployed. The particular stent configuration that is employed along with the material or materials selected for its construction determine the performance characteristics of the resulting device. Such characteristics include, but are not limited to, radial stiffness, longitudinal flexibility, longitudinal stability upon expansion, expansion ratio, coverage area, etc. Most stent configurations that are presently known have a fairly open structure with a commensurately limited coverage area. As a stent is expanded, a given area of support structure becomes distributed over an even greater area. This is especially aggravated in the event the stent configuration is selected so as to maintain a constant length during expansion.
The voids between the various support elements can be problematic. For example, lumen wall tissue directly adjacent to such voids is not supported and can prolapse. Similarly, plaque or other material not directly supported by a stent element could also prolapse or come loose and be swept downstream to cause an embolism. As a result, it is usually most desirable for a stent to provide as much coverage as possible.
Another problem encountered with heretofore known stent configurations is the risk of over-expanding the stent so as to unnecessarily traumatize or otherwise distort the body lumen at the deployment site. While a particular stent configuration may inherently limit the maximum diameter that can be achieved, such maximum can nonetheless exceed the maximum diameter that can be tolerated by the body lumen at the deployment site. Additionally, it may be desirable for certain portions of the stent to expand to a greater diameter than other portions of the stent. Conversely, it may be desirable for the stent to achieve a constant diameter over its entire length in contrast to the natural xe2x80x9cbow-tiexe2x80x9d effect many configurations are prone to as a stent""s resistance to expansion near its ends is often less than near its center and due to expansion characteristics of inflatable balloons at the balloon tapers.
The stent cover of the present invention overcomes the shortcomings of previously known covers. More specifically, the stent cover can be easily tailored to the unique requirements of a particular application. Such cover then serves to limit the expansion of the stent to either a preselected constant diameter or a preselected shape of varying diameter. Upon deployment of the stent with the cover in place, the prolapse of lumen wall tissue or plaque is positively precluded as is the escape of any embolic materials. Thus, the cover of the present invention is especially well suited for use in carotid artery applications. The healing of the deployment site is nonetheless not compromised as the cover of the present invention promotes the proliferation of endothelial tissue.
The cover of the present invention achieves the above-described advantages in that an elastomeric material is employed having very precisely defined pattern of elongated perforations or slits formed therein. The number, lengths and distribution of such perforations dictate the expansion characteristics of the material which in turn controls the ultimate expansion of the stent. Sections of the cover having an increased density of perforations or having perforations of longer length offer a reduced resistance to expansion while sections of low perforation density or shorter perforations more effectively resist expansion. Upon expansion, the perforations open into holes or orifices that promote the proliferation and ingrowth of endothelial tissue.
By selecting a perforation configuration other than what is essentially a one-dimensional slit, the stresses developed within the cover material upon expansion can be significantly reduced to preclude tearing. Additionally, by controlling the size and shape of the initially formed perforation, a more advantageously shaped and sized orifice is attainable upon expansion.
The stent cover of the present invention is manufactured either by perforating a tube of elastomeric material or by perforating a flat sheet of the material prior to forming it into a tubular configuration. The perforations may be formed by mechanical means or by laser cutting. By advantageously folding the material, multiple perforations are formed simultaneously. Cutting the material while in an expanded state allows more precisely defined openings to be formed.
These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment which, taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention.